A number of methods exist for transferring radio frequency (RF) signals through a window in an automobile. For example, an antenna can be attached to a window using an adhesive, and RF signals can be transferred to an apparatus on the inside of the window using capacitive coupling, slot coupling or aperture coupling. Typically, as is known in the art, the antenna system consists of a transmission line, a coupling device such as a capacitive plate, a slot, or an aperture, a matching circuit between the transmission line and the coupling device, and the antenna itself.
Known coupling methods require matching to the transmission line, that is, the RF signals transferred through the glass must be adapted to the particular transmission line, adding cost to the apparatus. Additionally, there is inherent signal loss due to coupling of approximately 1 dB or more, as well as an insertion loss due to the matching circuit.
Examples of prior vehicle antennas are disclosed in the U.S. Pat. Nos. 5,898,408 to Du, U.S. Pat. No. 4,882,592 to Studer, Jr. et al., U.S. Pat. No. 5,099,251 to Fisher, U.S. Pat. No. 4,799,098 to Blaese, U.S. Pat. No. 4,266,227 to Blaese, U.S. Pat. No. 4,109,251 to MacDougall, U.S. Pat. No. 5,850,199 to Wan et al., and U.S. Pat. No. 5,898,407 to Paulus et al.
Thus, there is a continuing need to provide an improved antenna system that reduces or eliminates signal loss caused by coupling.